1. Field of the Invention
This invention relates to hydrophilic porous membranes suitable for filtering and separating purposes in medical and industrial fields, and to a production process thereof.
2. Description of the Prior Art
Porous membranes are being used in medical applications including the separation of blood plasma, the filtration of infusion fluids, the separation of plasma proteins, the preparation of sterile water, and the like, as well as in industrial applications including the preparation of IC washing water and food processing water, the purification of other process water, and the like.
In recent years, large amounts of porous membranes have come to be utilized in water treatment devices for use in applications other than those described above, such as water purifiers for home and restaurant use.
In these applications, the dissolving-out of foreign matter from the material of the porous membrane would impair its safety or degrade the quality of the purified water or aqueous solution. Accordingly, there is a strong demand for porous membranes precluding the possibility of such dissolving-out of foreign matter.
A variety of porous membranes made of different materials and having different porous structures have been proposed. Among others, attention is now focused on porous membranes produced by melt-forming a crystalline thermoplastic polymer into a hollow fiber, tube or film, stretching the melt-formed product at a relatively low temperature to develop crazes in the amorphous regions between crystalline lamellae, and then hot-stretching the melt-formed product to create a porous structure therein. Since no additive or solvent is used, these porous membranes are suitable for use in applications prohibiting the dissolving-out of impurities and compounds therefrom. Hollow fiber membranes of this type are disclosed in Japanese Patent Laid-Open Nos. 137026/'77, 42919/'82 and 66114/'82, and flat films of this type are disclosed in U.S. Pat. No. 3,679,538 and Japanese Patent Publication No. 32531/'80.
Since porous membranes produced by the above-described processes consist solely of a polyolefin or fluorinated polyolefin that is essentially hydrophobic, it is difficult to use them directly for the purpose of filtering water-based liquids such as aqueous solutions. Accordingly, these porous membranes are usually used for the filtration of water-based liquids after being treated with hydrophilizing agents such as alcohols and surfactants.
One method for hydrophilizing hydrophobic membranes is disclosed in Japanese Patent Laid-Open No. 38333/'81. According to this method, a hydrophobic membrane is coated with a hydrophilic organic hydrocarbon monomer such as acrylic acid, methacrylic acid or vinyl acetate, and then exposed to ionizing radiation in a dose of about 1 to 10 megarads to fix the monomer chemically.
Another method for producing hydrophilic porous hollow fibers is disclosed in Japanese Patent Laid-Open No. 137208/'80. According to this method, a blend of two different polymers is melt-spun to form hollow fibers, which are stretched to cleave the interface between the different polymers. The resulting microporous hollow fibers are subjected to an after-treatment such as hydrolysis or sulfonation of the side-chain groups present in the constituent polymers. Thus, there are obtained hydrophilic porous hollow fibers having hydrophilized pore surfaces.
However, the hydrophilic porous membranes produced by the above-described methods have the following disadvantages.
Hydrophilizing treatment with an alcohol or a surfactant provides only tentative hydrophilization. Moreover, if the porous membrane having the hydrophilizing agent attached thereto is used for filtering or other purposes, the alcohol or surfactant migrates to and contaminates the purified water. Accordingly, it is necessary to wash off the hydrophilizing agent thoroughly before use. On this occasion, the pore surfaces will regain hydrophobicity if the washed porous membrane is dried. Thus, once the porous membrane is hydrophilized, it is necessary to replace the hydrophilizing agent with water and thereby keep its pore surfaces in contact with the water.
The method disclosed in Japanese Patent Laid-Open No. 38333/'81 can provide permanent hydrophilicity to the porous membrane, because the groups developing hydrophilicity are chemically fixed to the porous membrane. However, the need of exposure to ionizing radiation requires large-scale equipment, involves rather low process stability, and has a possibility of damage to the membrane material. Thus, it is difficult to manage and
In the method disclosed Japanese Patent Laid-Open control the process steps.
In the method desclosed Japanese Patent Laid-Open No. 137208/'80, the hollow fibers produced by melt-spinning a blend of different polymers and then stretching the spun fibers to make them porous generally have low porosity. Moreover, this method requires an after-treatment for hydrophilization, such as hydrolysis or sulfonation, which makes the process complicated.